At present methods of constructing large-size concave thin film mirrors are rather complicated, labor-intensive and expensive.
The method of constructing curved reflecting surfaces comprises placing a flat blank onto a matrix, then a specific mechanical and chemical treatment of the blank follows. This way is used to construct reflecting surfaces for the needs of optical industry, but it is not suitable for producing large-size thin film mirrors (Patent RU 2065616 C1 G 02 B 5/10).
Thin film is a kind of thin polymer film, polyethylene terephthalate, for example, or other suitable thin film. To give reflecting properties to the film one or both sides of its surface are coated with a layer of aluminum.
As a rule, concave thin film mirrors are constructed with the help of a suction chamber. The edges of the chamber are adapted for attaching the thin film and they lie in a desired invariable geometry. To imitate flight in ground-based simulators spherical mirrors are employed. In this case the suction chamber edges lie on the spherical surface. The thin film used for producing a mirror has the shape of a large flat sheet without initial tension. The film is attached to the edges of the suction chamber. Tension in the film is introduced while the air pressure in the suction chamber is reduced by a vacuum pump or when differential pressure is applied to the film surfaces forcing the film to be drawn into the suction chamber. An ideal shape will be achieved when the tensions in the thin film are close to uniform over the entire surface of the thin film. However in practice, while a large-size concave mirror is being formed an area in which the performance is not satisfactory inevitably arises in a border region of a thin film mirror, the border region being that portion of the thin film adjacent to the edge of the suction chamber. The tension in the film in the border region differs dramatically from the tension in the central region. This unusable border area is known as the mirror deadband. The irregular geometry of the mirror in the deadband makes it impossible to use the entire reflecting surface of the mirror. This in turn requires the thin film mirror to be constructed substantially larger, and causes raising its production cost.
There are methods elaborated to reduce the deadband by applying various tensioning means on the mirror. The most advanced method known on the priority date of the present invention is (A Method of Constructing a Thin Film Mirror, PCT WO 2007/007024 A1, Jun. 5, 2006), which includes the following:
(i) Attaching the thin film to the suction chamber, the suction chamber having edges which are adapted for attaching the thin film and which lie in a desired geometry for the thin film mirror to be constructed;
(ii) Applying a partial vacuum in the suction chamber such that tension is introduced into the thin film;
(iii) Adjusting the partial vacuum to form the thin film mirror but with the thin film mirror achieving the desired geometry only over a portion of the thin film mirror;
(iv) providing first tensioning means and locally adjusting the first tensioning means such that the portion of the thin film mirror which achieves the desired geometry is increased, the first tensioning means being in contact with the surface of the thin film adjacent to the edge of the suction chamber at a first distance which is substantially uniform from the edge of the suction chamber; and(v) providing second tensioning means locally adjusting the second tensioning means such that the portion of the thin film mirror which achieves the desired geometry is further increased, the second tensioning means at a second distance which is substantially uniform from the edge of the suction chamber, and which second distance from the edge of the suction chamber of the first tensioning means.
This method makes it possible to construct a large-size concave thin film mirror with a central part of a desired geometry.
However, the described above method of producing a thin film mirror does not provide mirrors without an area in which the performance is not satisfactory (the deadband). The deadband in a border region of a thin film mirror comprises a substantial portion of its surface which turns out useless and requires masking unless additional special-purpose means are used. The special-purpose tensioning means reduce the width of the deadband. Anyhow, even as an integral part of a finished mirror, those means do not eliminate the deadband completely, and the finished mirror in this case has larger geometrical sizes as compared to a mirror with a entirely reflecting surface. Moreover, installing additional tensioning means on a finished mirror lead to more expensive production costs.